Geoscience Reference
In-Depth Information
number of humans and the evolution of their settlements. The end of the gla-
ciations provoked changes in the main human activities; from hunting and
gathering to agriculture. Agricultural civilizations were also deeply in
uenced
by climatic variations. While warm periods were favourable to the spread of
cultivations and the multiplication of mankind, cold epochs corresponded to
demographic declines. Roman civilisation
ourished in a warm period and was
accompanied by population rise, while the early Middle Ages, characterized by
a cold climate, was an epoch of demographic decline. The so-called warm
Medieval Climatic Optimum coincided with worldwide population increase,
between 900 and about 1270, while the following Little Ice Age, from 1270
until 1820, was again a period of economic hardship and population stability or
slow increase. While present day energy systems heavily in
uence the envi-
ronment and climate, until a few centuries ago the opposite was true.
3. Ef
ciency and energy intensity Only a part of energy input is actually trans-
formed into useful energy (or energy services, that is mechanical work, light and
useful heat). How great this share is depends on the ef
ciency of the converters
of energy, that is labour (L) and capital goods (K). The thermodynamic ef
-
η
ciency (
) of the system of energy can be represented through the following ratio
between the energy services (Eu) and the total input of energy (Ei):
Eu
Ei
g ¼
Today, in our developed economies, this ratio is about 0.35; that is 35 % of the
input of energy becomes actual mechanical work, light or useful heat. In past
agricultural civilizations, the ef
ciency was much lower. A plausible calculation
is easier for the past, when biological converters prevailed, than for the present.
Today, in fact, the variety of machines, with diverse yields, make hard any
estimate. The ratio between useful mechanical work and input of energy into
biological converters, such as humans and working animals,
is around
20 %.
9
Part of the intake of energy in the form of food is not digested and is
expelled as waste, whilst the main part is utilized as metabolic energy in order to
repair the cells, digest and preserve body heat. A human being or animal con-
sumes even when inactive. The use of
15
-
rewood is even less ef
cient. The greater
part of the heat is dispersed without any bene
t for those who burn the wood. Its
yield is about 5
ciency of a vegetable energy system
based on biological converters, such as that of ancient civilizations, was around
15 % at
10 %. Overall, the ef
-
1,500 kcal. were transformed into useful
mechanical work or useful heat; the rest was lost. Thermal machines are much
more ef
the most:
that
is 1,000
-
cient than biological converters such as animals and humans.
4 Low Power Power is de
ned as the maximum of energy liberated in a second by
a biological or
technical engine.
In the economies of
the past another
9
See the useful Herman (
2007
).
